The invention relates to an electrical measurement apparatus and method for measuring an electrical characteristic of an earth formation, and in particular to an apparatus and method for measuring the electrical resistance of rock surrounding a drilled hole. The apparatus and method are likely to find their greatest utility in holes drilled for the exploration for hydrocarbons, and the following description will relate primarily to such use; however, it is to be understood that he invention can be used in other applications also.
It has been found that the electrical resistivity or resistance of rock surrounding a drilled hole can be used as a very good indicator of the structure of the rock, i.e. the electrical resistance is very sensitive to the structure of the rock. For example, rock itself, and rock containing oil, has a relatively high electrical resistance, whilst rock containing water and dissolved salts (e.g. brine) has a relatively low resistance.
Much work has been undertaken in recent decades to utilise the changes in electrical resistance adjacent to a drilled hole (and also deeper within the surrounding rock) to determine the structure of the rock, and in particular the likely presence of oil bearing strata therein.
Measurement tools and methods have been developed to measure the electrical resistance in the immediate vicinity of the drilled hole, i.e. within the few centimeters adjacent the drilled hole, and also deeper within the rock surrounding the drilled hole. The tools and methods for the latter type of measurements are typically less precise than those for the former measurements, i.e. in the latter case the measurements cover a larger volume of rock and are therefore less sensitive to small variations within only a part of that rock. The present invention is particularly suited to the former measurements, and is intended to provide very precise measurements (though it could, if desired, also be utilized with the latter measurements).
It is a requirement of measurements in the immediate vicinity of the drilled hole that the measurements be as precise as possible, with a resolution, for example, of as little as 0.1 inches (approximately 2.5 mm).
It will be understood that when measurements are to be taken within a drilled hole, the measurement tool is first introduced into the hole, and moved to the distal end of the zone of interest. The tool is typically connected by a cable to a winch at the surface, and the measurements are taken as the tool is pulled out of the hole, past the region of interest. It is not economic to allow the tool to stop for each measurement, nor is it desirable since if the tool stops it is likely to stick in position (within the mud which will typically be present within the hole, which mud can readily cause the tool to stick to the wall of the hole). A typical rate of movement of the tool during measurement is approximately 30 feet per minute (approximately 0.167 meters/second), and this speed is generally accepted as a realistic compromise between economics (the desire to take the measurements as quickly as possible so that, for example, drilling can subsequently be continued), the ability to take sufficient measurements sufficiently quickly, and the likelihood of the tool sticking.
To take a measurement every 0.1 inches whilst the tool is travelling at 30 feet per minute requires a measurement to be taken every one sixtieth of a second.
It is also known that in more precise measurements several sensor elements can be arranged on a single sensor pad, a measurement being taken from each sensor element. A known design of sensor pad has twenty five sensor elements, for example.
When taking resistance measurements in these applications, it is typical to utilize an alternating applied voltage. This has the advantage that electrolytic and other contact-induced electrical effects between the sensor elements and the rock can be ignored, it being understood that those effects induce DC voltages, or at least voltages which are sufficiently invariant to be considered to be DC. There is, however, a practical upper limit to the frequency which can be used, since higher frequencies attenuate more within the rock, and are prone to phase shifts between the applied voltage and measured current. Generally, frequencies in the range from 5 kHz to 20 kHz can be used, with the embodiment described herein using a frequency of around 7.5 kHz.
FIG. 1 demonstrates the principle involved in taking an electrical resistance measurement of the rock surrounding a drilled hole, which principle underlies the measurement methods used in many prior art applications, and also within the present invention. In FIG. 1, a hole 2 has been drilled within formation 4. A measurement tool (not shown) has been inserted into the hole, and includes a sensor element 6, which sensor element is surrounded by a guard element 8. The sensor element 6 and guard element 8 are connected to a voltage generator 10, supplying an alternating voltage. The electrical circuit is completed by an electrode 12 in contact with the rock remote from the sensor element 6 and guard element 8. Since the electrode 12 is remote from the sensor element 6 and guard element 8, it is typically considered as electrical infinity.
FIG. 1 also shows, in dashed outline, a representation of the current flow through the rock, i.e. between the sensor element 6 and guard element 8, and the electrode 12. It is desired that the electrode 12 be sufficiently far from the sensor element 6 that the current flow is substantially perpendicular to the rock surface for a distance within the rock, so that the current flow through an imaginary cylinder 14 is substantially linear and uniform.
If the current flow through the imaginary cylinder 14 is linear and uniform, the current flowing through the cylinder will be dependent upon the electrical resistance of the rock within the cylinder 14, and this current corresponds to the current flowing through the line 16.
It will be understood that the guard element 8 serves to reduce (and hopefully eliminate) the edge effects of the sensor element 6. It is desired that the voltage of the guard element 8 matches the voltage of the sensor element 6 at all times, so that no current flows through the rock between the sensor element and the guard element. This will also help to ensure that the current flowing though the imaginary cylinder 14 is linear and uniform.
The design of the sensor element and guard element, as well as other characteristics of the apparatus, which seeks to ensure that the current flow is substantially perpendicular to the rock surface adjacent the sensor element 6 is known in this art as xe2x80x9cfocussingxe2x80x9d, and a properly focussed apparatus can be used to determine the current flow through the imaginary cylinder 14 by determining the current flow through the line 16.
From a measurement of the current flowing through the line 16, and a knowledge of the voltage applied by the generator 10, the resistance of the electrical circuit (including the rock within the imaginary cylinder) can be determined from Ohms law. However, it is necessary to apply a calibration factor so that the resistance of the rock within the imaginary cylinder 14 can be determined. The calibration factor will depend upon the particular apparatus, and in particular its geometry and componentry, but once established for a tool will not change unless the apparatus geometry or componentry is changed. In addition, absolute resistance values for the formation 4 are seldom required, but variations in the resistance at different locations within the formation are particularly useful.
Early workers in this field utilized a resistor in the line 16 so that a voltage drop across the resistor could be measured and the current flow calculated. However, that method had the disadvantage that the resistor caused a difference in the voltage between the sensor element and the guard element, adversely affecting the focussing of the apparatus. The voltage drop need only be very small, e.g. a few thousandths of the guard voltage, to adversely affect the focussing.
U.S. Pat. No. 4,468,623 utilizes a xe2x80x9ctransformerxe2x80x9d, i.e. a separate circuit adjacent the line 16 in which a current is induced, dependent upon the current flow in the line 16. However, even though there is no physical contact between the line 16 and the separate circuit, the transformer would nevertheless cause a voltage drop in the line 16, and could also cause an unwanted phase shift in the current.
Once the current through the sensor element has been detected (more or less accurately, depending upon the method utilized), its value must be calculated and output so that it may be utilized to generate information about the electrical resistance of the rock within the region of interest. However, the signal containing the information about the current flow will typically include a certain amount of noise, and previously it has been sought to reduce the amount of noise by filtering methods.
A perfect filter which removes all of the noise and retains all of the signal is not achievable in practice, and any filtering method will lose some of the signal. Furthermore, a filter will be corrupted to some extent by the continuous nature of the signal, i.e. a filter is not reset between calculations being made upon the signal, and in the case of widely fluctuating signals the filter might contribute to the calculated signal by retaining part of a previous signal. This is particularly important when the short time available for each measurement, whilst the tool is moving to the next measurement position, is consideredxe2x80x94since successive measurement signals can vary widely and the time to take a measurement is very short, the xe2x80x9cmemory effectxe2x80x9d is potentially severe.
The invention seeks to reduce or avoid the above-stated disadvantages with the prior art apparatuses and methods.
According to one aspect of the invention, the apparatus includes a detection portion including an op amp (operational amplifier) acting in xe2x80x9cvoltage followerxe2x80x9d mode, and with a resistor in the feedback loop. It will be understood that the op amp in this mode will drive whatever current is necessary (within the limits of the power supply and circuit) to ensure that the voltage at the output (to which the sensor element is connected) will always be equal to the voltage at the input (to which the generator is connected). The resistor is located within the feedback loop between the op amp and the sensor element. The voltage drop across the resistor can be measured, and the current flowing through the resistor (and hence from the sensor element and into the rock) can be calculated. The resistor can be of relatively large value so that the voltage drop is relatively large and readily measurable.
Preferably, there is a plurality of sensor elements. It is not usually practical to provide a separate op amp for each sensor element, and the apparatus can include means to switch a single op amp between the sensor elements in sequence, so that the outputs from the sensor elements are multiplexed.
The apparatus also includes a calculation portion, the purpose of which is to provide an output signal corresponding to the current flow measured by the detection portion. According to another aspect of the invention the apparatus includes an integrator within the calculation portion.
An integrator provides an output signal indicative of the current flow. The integrator can be reset to a datum value (i.e. zero) after each measurement calculation, so that no memory of the previous calculation is retained. In addition, the integrator can virtually eliminate the noise whilst retaining all of the signal, since the value of the noise will alternately add to and subtract from the underlying signal, in approximately equal amounts, so that the noise can be effectively summed substantially to zero.